1. FIELD OF THE INVENTION
The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid (hereinafter referred to as 2,6-NDCA). 2,6-NDCA is a useful compound serving as a raw material for highly functional resins such as polyethylene naphthalete (PEN resin), or the like.
2. DESCRIPTION OF THE RELATED ART
It is known that various processes are available for oxidizing 2,6-dimethylnaphthalene (hereinafter referred to as 2,6-DMN) by molecular oxygen in the presence of a catalyst including cobalt, manganese and bromine so as to produce 2,6-NDCA. For example, such processes are disclosed in Japanese Patent Laid-Open Nos. 48-34153 and 49-42564. However, they present the following problems. Unless a molar ratio of 2,6-DHN to acetic acid in the reaction system is maintained at a low level such as 1/100 or lower, intermediate oxidation products such as 6-formyl-2-naphthoic acid are increased, thus preventing high yields of 2,6-NDCA. Also, other processes are known such as a two-step oxidizing process where the temperature of the oxidation reaction is varried (Japanese Patent Publication No. 59-13495) and a process where the oxidizing is carried out in a solvent mixture of aliphatic carboxylic acid and an aromatic compound (Japanese Patent Laid-Open No. 64-3148). However, from the industrial point of view, the provision of a process for economically producing a high yield of 2,6-NDCA by using a small amount of a catalyst is desirable.